For the conversion of inputted image data represented by one color space into output image data represented by another color space, there has been proposed a method of color conversion employing direct mapping or interpolation operations.
Direct mapping is a method of color conversion using a three-dimensional LUT (Look Up Table, operational memory), which is a table of correspondence between color space data of inputted image data and color space data of output image data.
However, since storing all corresponding output image data of inputted image data requires a considerable memory space, the three dimensional LUT is generally thinned out and three dimensional interpolation operations are performed to complement the table.
However, a problem of three dimensional interpolation operations is that, in the vicinity of color space borders, it requires data of grid points outside of the color space. As a result, accuracy of color conversion suffers.
As a countermeasure, there has been proposed a technique as described in Patent Publication 1. In Patent Publication 1, interpolations are performed only with the grid points that reside in a cubic or rectangular color space. For example, an eight-point interpolation is performed when all of the 8 grid points making up a grid corresponding to the input data reside in the color space. Otherwise, a four-point interpolation is performed that uses only data of grid points contained in the color space. That is, the technique changes the method by which interpolations are performed.
However, changing the method of interpolation is disadvantageous because each method uses different accessing patterns to the memory or requires different operation procedures. This requires more than one interpolation operation circuit for these different interpolation processes, with the result that cost is increased.
Further, Patent Publication 1 teaches storing distances between grid points, and shifting positions of the grid points by the equivalent distance in the vicinity of color space borders, so that interpolation operations are performed by regarding these border areas as an outermost edge of the color space, without using data of grid points that reside outside of the color space. However, since the shape of gamut (color space) is complex, a large data volume is required to store positions, and access to the data is rather complex.
Further, while the gamut that can be processed in the input and output devices of the image processing apparatus is usually spherical (ellipsoidal, football-like shape) in shape, the color space used in the calculations of three-dimensional interpolations is cubic or rectangular. Thus, if the spherical gamut were to be enveloped by the cubic or rectangular color space the color space would contain a large number of grid points that fall outside of the gamut. That is, the color space would contain a large number of grid points that are not used for the interpolation operations. In other words, when the data of the color space is to be used for the interpolation operations, an operational memory needs to store data of grid points that are not actually used for the interpolation operations. As a result, an unnecessarily large memory space is used and it has adverse effects on the processing capability of the image processing apparatus.
[Patent Publication 1]
Japanese Laid-Open Patent Publication No. 2001-257899 (published on Sep. 21, 2001)
[Patent Publication 2]
Japanese Laid-Open Patent Publication No. 2005-167503 (published on Jun. 23, 2005)
Patent Publication 2 discloses a technique for solving the foregoing problems. The technique disclosed in Patent Publication 2 is intended to improve processing capability of the image processing apparatus as follows. In this technique, a color space is set that corresponds in shape to the gamut that is reconstructable in the input and output devices of the image processing apparatus, and only the grid points contained in this color space are stored in the memory to reduce the number of grid points developed in the memory and thereby save memory space.
However, the technique as disclosed in Patent Publication 2 is merely optimization of a color space according to a gamut that is reconstructable in the input and output devices of the image processing apparatus. More specifically, the interpolation of the inputted image using the color space uses the grid points that reside in the gamut of the inputted image. However, since the gamut of the inputted image is confined in and smaller than the gamut that is reconstructable in the input and output devices of the image processing apparatus, the color space disclosed in Patent Publication 2 also requires the memory to store data of grid points that are not used for the interpolations, with the result that unnecessary memory space is used.